1. Field of the Invention
The present invention relates generally to a cell projection type electron beam lithography system and a pattern writing method. More specifically, the invention relates to an electron beam lithography system and pattern writing method, which are suitable for pattern writing using a low-energy electron beam.
2. Related Background Art
Electron beam lithography systems for writing patterns on the surface of substrates, such as semiconductor wafers, using an electron beam have particularly excellent resolution, so that the systems are widely noticed as systems meeting the demands that semiconductor circuits be further scaled down.
It is pointed out that electron beam lithography systems, which are put to practical use at present, have disadvantages that (1) the precision of patterns is bad due to the influence of the proximity effect, (2) throughput is low, and so forth.
The proximity effect is a phenomenon that electrons being incident at a high acceleration voltage are scattered in a resist on a wafer, and back-scattered electrons cause the resist around the incident point to be exposed, so that the precision of a pattern deteriorates. In order to correct this, it is required to correct dose in accordance with the layout of the pattern, so that the construction of the system is very complicated.
One of causes for low throughput is that the sensitivity of the resist is low. The reason for this is that the sensitivity of the resist depends on the quantity of secondary electrons produced by the incidence of an electron beam, whereas if an electron beam is incident at a high acceleration voltage, the incident electrons pass through the resist, so that required secondary electrons are not sufficiently produced.
As a technique capable of solving the above described two problems, there is a low acceleration electron beam lithography system. The low acceleration herein indicates an acceleration voltage capable of ignoring the influence of the proximity effect, specifically about 5 kV or less. Because, if the acceleration voltage is low, the energy of incident electrons is low, and the influence of back scattered electrons is small, so that the proximity effect can be reduced. In addition, the energy of incident electrons is low, the scattering cross section in the resist is large, so that the production efficiency of secondary electrons is high. Since low-energy secondary electrons contribute to the sensitizing of the resist, the improvement of the production efficiency of secondary electrons directly appears as the improvement of the sensitivity of the resist. The improvement of the sensitivity of the resist directly appears as the improvement of throughput.
Thus, the low acceleration electron beam lithography system has greater advantages than the high acceleration system, and has remarkable advantages particularly in the field of the direct writing. Moreover, as another advantage obtained by adopting the low acceleration electron beam lithography system, there is an advantage in that an electrostatic column (an electron optical system using an electric-field lens) can be used if the acceleration voltage is low. The electrostatic column has the merits of being easy to be miniaturized, and of having good response characteristics since no hysteresis is caused unlike magnetic-field lenses because an electric-field lens is used therein. However, on the other hand, there is a disadvantage in that the electrostatic column has a high lens voltage when it is used as a focussing lens. For example, when the acceleration voltage is 50 kV, the lens voltage is in the range of from 70 to 100 kV. This is of no practical use. However, if the acceleration voltage is low (5 kV or less), the lens voltage is about 10 kV. This can be put to practical use in the existing high voltage power supply technique. The electrostatic lens has been put to practical use as a deflecting system requiring high speed characteristics, not as a focussing lens. However, electron beam lithography systems where all lenses are constructed of electrostatic lenses have not been put to practical use.
Thus, according to the low acceleration electron beam lithography technique, there is some possibility of realizing a direct writing having high throughput, and there is some possibility of realizing a small lithography system having high controllability by electrostatic columns. It is expected that this is very effective in the formation of a pattern scaled down in future.
However, with respect to the low acceleration electron beam lithography system, there is a problem of the space-charge effect. The space-charge effect is a phenomenon that the flow of charged particles is restricted by the space potential formed by space charges. In the electron beam lithography system, the space-charge effect appears as the restriction of the emission current from a cathode and as the broadening of the beam diameter caused during the convergence of the electron beam including crossovers (so-called beam blurring). Since the space-charge effect depends on the energy of an electron beam, the space-charge effect conspicuously appears when the energy of electron beam is low. Therefore, the space-charge effect causes serious problems in the low acceleration electron beam column.
On the other hand, a cell projection type electron beam lithography system using electrostatic columns has been proposed by H. Sunaoshietal. (Jpn. J. Appl. Phys. Vol. 34 (1995), pp. 6679-6683 (which will be hereinafter referred to as xe2x80x9cBackground Art 1xe2x80x9d)). Another cell projection type lithography system is disclosed by K. Hattori et. Al (J. Vac. Scl. Technol. B 11(6), November/December 1993, p2346 (which will be hereinafter referred to as xe2x80x9cBackground Art 2xe2x80x9d)).
As shown in Background Art 2, a projection optical system is a double-reduction lens system comprising a reduction lens and an objective lens. The beam blurring caused by the space-charge effect occurs in the projection optical system and on the top surface of the substrate.
With respect to an electrostatic lens, two modes can be selected: one mode is a deceleration mode and the other mode is an acceleration mode. In general, the aberration performance of the acceleration mode is superior to that of the deceleration mode. However, the acceleration mode is of no practical use since the electric field of the lens must be a high electric field. Therefore, the deceleration mode is generally used. In the deceleration mode, after electrons are once decelerated in the lens, the electrons are accelerated again to be emitted from the lens.
An Electron beam establishes a crossover by the reduction lens once to be further decelerated in the objective lens. At this time, the electron beam is greatly affected by the space-charge effect.
Moreover, the surface of the substrate is most remarkably affected by the space-charge effect. In this portion, the beam blurring caused by the space-charge effect depends on the aperture angle and the focal length (the distance where electrons travel in a field-free space). That is, the influence of the space-charge effect increases as the aperture angle decreases, and the influence of the space-charge effect increases as the focal length increases. However, if the aperture angle is increased in order to reduce the influence of the space-charge effect, the influence of aberration increases. In particular, the influence of chromatic aberration increases when operation is carried out at a low acceleration voltage. Therefore, the aperture angle can not be so great. This is a serious problem when an electron optical system for a low acceleration electron beam lithography system is designed.
The influence of the space-charge effect on the surface of a substrate has been analyzed by Y. Yamazaki and M. Miyoshi (Optik 96, No. 4 (1994), pp. 184-186 (which will be hereinafter referred to as xe2x80x9cBackground Art 3xe2x80x9d)). This has analyzed the relationship between the increasing ratio of the beam diameter and the aperture angle ratio due to the space-charge effect of an elliptical cross-section beam. More specifically, this has analyzed the influence of the space-charge effect appearing in an optical system when the image of an elliptical cross-section beam having a uniform current density is formed at a circular spot on the surface of a substrate on the anamorphic image-formation optical conditions.
The relationship between the increasing ratio of the beam diameter and the aperture angle due to the space-charge effect is shown in FIG. 1. Since the trajectories in X-axis and Y axis directions are the same in a rotation symmetric system, the aperture angles of incident electrons are the same as each other, so that the aperture angle ratio is 1 (xcex1/xcex3=1). The graph of FIG. 1 shows the decreasing rate of the broadening of a beam due to the space-charge effect, which varies in accordance with the increase of the aperture angle ratio, assuming that the broadening of a beam due to the space-charge effect is 1 in the case of a rotation symmetric system, i.e., in the case of xcex1/xcex3=1. It can be seen from this graph that the increasing ratio of the beam diameter decreases to reduce the beam blurring as the aperture angle ratio leaves xcex1/xcex3=xcex3/=1.
This shows that as the aperture angle ratio increases, the aspect ratio of an elliptical cross-section beam in an equipotential space increases, and the intensity of the electric field acting on electrons at the outermost edge of the beam flux decreases, so that the influence on the beam diameter due to the space-charge effect is suppressed. For example, when the aperture angle ratio is 10, it can be seen that the intensity of the electric field acting the electrons at the outermost edge is smaller by 58% than that in the case of a circular cross-section beam, as shown by broken lines in the figure. In proportion thereto, the increase of the beam diameter is suppressed.
Therefore, it can be seen that it is possible to provide an electron optical lens column capable of remarkably reducing the beam blurring due to the space-charge effect if it is possible to form an optical system, which can form an isotropic CP aperture image on a substrate by means of an elliptical cross-section beam and which can increase the aspect ratio of the elliptical cross-section, i.e., an optical system capable of increasing the incident angle ratio on the surface of a substrate.
It has been described in some papers that a multipole asymmetrical lens system represented by a quadrupole lens is used for forming a probe. Some applied examples of such multipole asymmetrical lens systems are described in Electron Microscope (Vol. 25, No. 3 (1990), pp. 159-166 (which will be hereinafter referred to as xe2x80x9cBackground Art 4xe2x80x9d)) and Electron Microscope (Vol. 25, No. 1 (1991), pp. 58-65 (which will be hereinafter referred to as xe2x80x9cBackground Art 5xe2x80x9d)). On page 159 of Background Art 4, there is shown a conceptual diagram of an electron optical system of a variable shaped type electron beam lithography system using three-stage electric-field quadrupole lenses. Referring to FIG. 2, the construction of this electron optical system will be described below. Furthermore, in the following figures, the same reference numbers are given to the same elements, and the descriptions thereof are suitably omitted.
The electron optical system of a variable shaped beam system shown in FIG. 2 is an electron optical system for irradiating a rectangular diaphragm 73 with an electron beam, which is emitted from an electron gun 71, by means of an electromagnetic lens (a condenser lens 72) to control the demagnifications in X and Y directions of the rectangular beam which passes through the rectangular diaphragm 73, by means of three-stage quadrupole lenses 74. Octpole lenses 75 are used for correcting astigmatism and deflecting the electron beam. As can be seen from FIG. 2, the electron optical system described in Background Art 4 comprises lenses of a rotation symmetric system in an illumination optical system, from the electron gun to the condenser lens. A projection optical system comprises an asymmetric lens system comprising the quadrupole lens system 74 and the octpole lenses 75.
Since the upper half of the electron optical system, i.e., the illumination system, is the rotation symmetric system, the outgoing angles of the trajectories in the X and Y directions of electrons emitted from the rectangular aperture 73 are equal to each other (xcex1=xcex3). Therefore, when the rectangular aperture image is simply reduced and projected, the incident angles on the surface of the substrate are equal to each other. That is, the demagnification in the X-axis directions and the demagnification in the Y-axis directions are equal to each other. Therefore, assuming that the incident angle in the X-axis directions is xcex12 and that the incident angle in the Y-axis directions is xcex32, xcex12=xcex32 is obtained.
The electron optical system described in Background Art 4 aims at providing an electron beam lithography system of a variable shaped beam system. Therefore, the electron optical system changes the shape of an irradiation surface on a substrate, which is irradiated with an electron beam, by optionally changing the demagnifications in the X and Y directions with respect to the isotropic rectangular aperture image. When a linear beam is projected, the demagnifications in the X and Y directions are changed. Specifically, as described in Background Art 4, the excitation conditions of an electrodes Q1 through Q3 of the three-stage quadrupole lenses are fixed, and the excitation conditions of the electrodes Q2 and Q3 are changed. Thus, the demagnifications in the X-axis and Y-axis directions are different from each other, so that the incident angles on the surface of the substrate are also different from each other. Therefore, assuming that the incident angle in the X-axis directions is xcex1xe2x80x2 and that the incident angle in the Y-axis directions is xcex3xe2x80x2, xcex1xe2x80x2xe2x89xa0xcex3xe2x80x2 is obtained.
An image forming optical system based on different incident angles is disclosed by Y. Yamazaki and M. Miyoshi (Nuclear Instruments and Methods In Physics Research A363 (1995), 67-72 (Background Art 6)). In this paper, there is described the image forming conditions of an electric-field quadrupole lens system for reducing and projecting an electron beam from a line cathode (exactly a rectangular cathode) to an isotropic circular beam. The electron optical system shown herein comprises a light source of 10 xcexcmxc3x97100 xcexcm, an LaB6 line cathode having an aspect ratio of 10, and an anamorphic image forming system of a three-stage quadrupole lens system (triplet). This electron optical system is designed to deform and reduce the rectangular beam into the circular beam by setting a demagnification of 1/1000 in the X-axis (minor axis) and a demagnification of 1/100 in the Y-axis (major axis).
The ray diagram of the triplet image forming optical system of the quadrupole lens described in Background Art 6 is shown in FIG. 3. The major axis of the rectangular cathode corresponds to X-axis, and the minor axis thereof corresponds to Y-axis. This figure shows two image forming conditions based on the excitation conditions of the quadrupole lens electrode Q2. In order to satisfy the reduction ratio, the strong excitation mode shown by the solid line in the figure is the optimum solution. An X trajectory 81, which is a trajectory in the direction of a major axis, is a convergent trajectory in the quadrupole lens electrode Q1, a divergent trajectory in the quadrupole lens electrode Q2, and finally converges in the quadrupole lens electrode Q3 to form an image on the top surface of a substrate 21. On the other hand, a Y trajectory 82, which is a trajectory in the direction of a minor axis, is a divergent trajectory in the quadrupole lens electrode Q1, a convergent trajectory in the quadrupole lens electrode Q2, and finally forms an image while diverging in the quadrupole lens electrode Q3.
Since the magnificatios on the X and Y axes are different from each other in order to form the image of an electron beam, which is emitted from an asymmetric rectangular cathode, to be an isotropic round beam, the incident angles on the surface of the substrate are different in accordance with the demagnification (p68, FIG. 15). According to this Background Art 6, Mx (demagnification of X-axis trajectory)=1/1000, and My (demagnification of Y-axis trajectory)=1/100, so that the ratio of incident angles is 1:10. Thus, although Background Art 6 discloses the basic concept concerning the construction of the image forming optical system for forming the image of the electron beam, which is emitted from the asymmetric rectangular cathode, to be the isotropic round beam, it does not disclose any concrete means for applying the image forming optical system to a cell projection type electron optical system.
It is therefore a first object of the present invention to provide a low acceleration electron beam lithography system wherein the influence of the space-charge effect is reduced to provide excellent resolution and throughput.
It is a second object of the present invention to provide a method for writing a pattern at a high resolution and a high throughput while greatly reducing the influence of the space-charge effect using a low acceleration electron beam.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided:
an electron beam lithography system having an electron optical system comprising: an electron beam emitting device for emitting an electron beam to a substrate on which a desired pattern is to be written, the electron beam emitting device having a cathode for emitting electrons, the electron beam having a cross section, which is asymmetric with respect to an optical axis, and, the cathode emitting at an acceleration voltage at which the quantity of back scattered electrons generated from the substrate by irradiation with the electron beam is lower than a quantity at which the light exposure of a close pattern to be written is affected; a character aperture including a hole having a shape corresponding to the shape of the desired pattern; an illumination optical system for controlling the emitted electron beam so as to irradiate the character aperture with the electron beam, the illumination optical system being set with demagnifications which are different in X-axis and Y-axis directions to each other when the direction of the optical axis is Z-axis direction so that the character aperture is irradiated with the electron beam which include a cross section having a first aspect ratio of about 1, the electron beam being shaped so as to correspond to the shape of the shaping aperture, and a projection optical system for demagnifying the electron beam and for forming an image on the substrate, the projection optical system demagnifying the electron beam at the same demagnification in X-axis and Y-axis directions to each other and forming the image on the substrate through a trajectory which is asymmetric with respect to the Z-axis at different incident angles in the X-axis and Y-axis directions to each other.
According to the above described electron beam lithography system, the demagnification which is asymmetric with respect to the optical axis is the in the illumination optical system, so that the electron beam are incident on the shaping aperture at different incident angles in the X and Y directions. At this time, the illumination optical system is generally operated on the image forming condition, i.e., on the condition that the beam trajectories in the X and Y directions intersect each other at one point on the optical axis. In a rotation symmetric optical system, the image forming condition is always established, whereas in an asymmetric optional system, the image forming condition is established when the focal point in the X direction (a point at which the X trajectory intersects Z-axis) is coincident with the focal point in the Y direction (a point at which the Y trajectory intersects Z axis). The illumination optical system originally aims at illuminating the CP aperture, so that it is not always required to form the image according to the present invention. Therefore, the electron beam asymmetrically emitted from the light source is incident on the shaping aperture so as to have an isotropic cross section (an aspect ratio of about 1), so that the electron beam having a cross section (a CP aperture image) corresponding to the shape of the hole of the aperture is emitted from the hole at the same outgoing angle as the incident angle on the shaping aperture. In addition, the projection optical system reduces the electron beam, which is emitted from the hole, at the same demagnification in the X and Y directions, and forms the image on the surface of the substrate at different incident angles in the X and Y directions. Thus, the electron beam is reduced through the trajectory which is asymmetric with respect to the optical axis to form an image isotropically on the surface of the substrate without establishing any crossovers in the illumination optical system and projection optical system. As a result, the influence of the space-charge effect in the projection optical system is removed, so that it is possible to greatly reduce the influence of the space-charge effect on the surface of the substrate. Thus, it is possible to greatly reduce the beam blurring which is caused by conventional low acceleration electron beam lithography systems. As a result, it is possible to provide an electron beam lithography system having excellent resolution and high throughput while using a low-energy electron beam.
In the above described electron beam lithography system, the illumination optical system preferably has a first multipole lens.
Moreover, the illumination optical system preferably has at least two stages of the first multipole lens, and the image forming conditions of the first multipole lens are preferably set so that the first multipole lens serves as an extension system in the X-axis direction and a reduction system in the Y-axis direction in accordance with the first aspect ratio.
The methods for forming the electron beam having the asymmetric cross section include two methods: one is a method for emitting electrons from the rectangular cathode at the same outgoing angle in the X and Y directions, and the other is a method for emitting electrons from the circular cathode at different outgoing angles in the X and Y directions. In either method, the same effect can be obtained by the asymmetric optical system comprising the multipole lens system.
That is, when the rectangular cathode is used, the electrons emitted from the electron beam emitting device at the same outgoing angle in the X and Y directions are incident on the shaping aperture at different incident angles in the X and Y directions.
On the other hand, when the circular cathode is used, the electrons emitted from the electron beam emitting device at different outgoing angles in the X and Y directions are incident on the shaping aperture at optional different incident angles in the X and Y directions through the trajectory which is asymmetric with respect to the optical axis. As a result, the electron beam is incident on the shaping aperture without establishing any crossovers.
The rectangular cathode is preferably formed of lanthanum hexaboride (LaB6), and the circular cathode is preferably a thermal-assisted field emission type cathode.
In the above described electron beam lithography system, the projection optical system preferably has a second multipole lens.
Moreover, the projection optical system preferably has four stages of the second multipole lens, and the image forming conditions of the multipole lens are set so that the electron beam passes through a trajectory which repeats divergence and convergence in the X-axis direction and which repeats convergence and divergence in the Y-axis direction contrary to the X-axis direction.
Thus, the electron beam having a cross section corresponding to the shape of the hole of the aperture is imaged on the surface of the substrate without establishing any crossovers in the projection optical system. As a result, the influence of the space-charge effect in the projection optical system is removed, and the influence of the space-charge effect on the surface of the substrate is greatly reduced, so that it is possible to greatly reduce the beam blurring.
The above described multipole lens may include an electrostatic quadrupole lens or an octpole lens.
According to the second aspect of the invention there is provided:
a pattern writing method for writing a desired pattern on a substrate using an electron beam lithography system comprising an electron optical system which includes an electron gun for emitting an electron beam, a character aperture having a hole having a shape corresponding to the shape of the desired pattern, an illumination optical system for controlling the emitted electron beam so as to irradiate the character aperture with the electron beam, and a projection optical system for controlling the electron beam from the shaping aperture in the shape in accordance with the shape of the hole so as to form an image on the surface of the substrate, the pattern writing method comprising: a first step of emitting the electron beam having a cross section which is asymmetric with respect to an optical axis at an acceleration voltage at which the quantity of back scattered electrons generated from the substrate by irradiation with the electron beam is lower than a quantity at which the light exposure of a close pattern to be written is affected; a second step of setting demagnifications of the illumination optical system to be different in X-axis and Y-axis directions to each other when the direction of the optical axis is Z-axis direction, and of controlling the electron beam so that the electron beam is cast on the character aperture so as to have a substantially isotropic cross section; and a third step of controlling the projection optical system so as to demagnify the electron beam from the character aperture at the same demagnification in X-axis and Y-axis directions and so as to cause the electron beam to pass through a trajectory which is asymmetric with respect to the optical axis and to be cast on the substrate at different incident angles in the X-axis and Y-axis directions to each other to form an image on the substrate.
The second step preferably includes a step of setting focal conditions so that the electron beam is cast on the shaping aperture at different incident angles in the X-axis and Y-axis directions to each other.
Preferably, the step of setting the focal conditions is a step of setting focal conditions in the X-axis and Y-axis directions independently of each other so that the focal point of the electron beam in the X-axis direction is different from the focal point of the electron beam in the Y-axis direction in order to prevent the electron beam from establishing any crossovers. Thus, an optical system, which is asymmetric with respect to the beam axis, is also formed in the illumination optical system, so that it is possible to avoid the crossovers of the electron beam.
The third step preferably includes a step of setting image forming conditions in the projection optical system so that the electron beam passes through a trajectory which repeats divergence and convergence in the X-axis direction and which repeats convergence and divergence in the Y-axis direction contrary to the X-axis direction.
The electron gun preferably has a rectangular cathode having an aspect ratio of a value other than 1 in the X-axis and Y-axis directions, and the first step preferably includes a step of emitting electrons from the rectangular cathode at the same outgoing angle.
Alternatively, the electron gun may have a substantially circular cathode, and the first step may include a step of emitting electrons at different outgoing angles in the X-axis and Y-axis directions to each other. By this step, the same effect can be obtained.